Some forms of electronic communication systems encode digital data on multiple carrier frequencies using a technique known as orthogonal frequency division multiplexing (OFDM) in which multiple closely spaced orthogonal sub-carrier signals with overlapping spectra are emitted to carry data. In addition, in some forms of electronic communications systems, multiple streams of data (“layers”) are transmitted in parallel in an arrangement sometimes referred to as multiple-in-multiple-out (MIMO). In some circumstances, frequency division multiplexing (FDM) is used to transmit these multiple streams of data, where non-overlapping frequency bands are used to carry separate signals.
In order for the receiver to perform channel estimation for each layer, a demodulation reference signal (DMRS) may include multiple “ports” (one for each layer), where these ports are orthogonal to each other. This orthogonality can be achieved by using frequency division multiplexing (FDM), where different subcarriers are used in the frequency domain, or by using code division multiplexing (CDM) which uses orthogonal cover codes (OCC) in the time domain (TD-OCC) or frequency domain (FD-OCC).
Such modulation and demodulation techniques may be applied, for example, in wireless cellular communications technologies such as Long-Term Evolution (LTE®) and 5G New Radio (NR).